When wearing contact lenses, the amount of oxygen supplied to the surface of the eye from the air decreases. Clinical results have linked this to inhibited mitosis of corneal epithelial cells and corneal thickening. Accordingly, to achieve a highly safe contact lens, attempts have been made to improve the oxygen permeability of contact lens materials (also referred to simply as “materials” hereinafter).
Since the materials in hydrated soft contact lenses are soft, such lenses are generally known to be quite comfortable to wear. However, the oxygen permeability of conventional lenses depends on the water content of the lens. Thus, attempts have been made to increase the water content of the materials. However, there is a limit to how much the oxygen permeability can be enhanced by simply increasing the water content of the materials. Accordingly, silicone hydrogels, obtained by polymerizing monomers such as silicone monomers and siloxane macromonomers, have recently been developed as a method of increasing oxygen permeability independently of the water content. These have also been developed into lens products.
Since uniform products can be manufactured in large quantities, the cast molding manufacturing method is generally employed to manufacture soft contact lenses. Polypropylene is often employed as the material of the mold used in the cast mold manufacturing method because of low cost and ease of molding. However, polypropylene is itself a hydrophobic material. Thus, when manufacturing a silicone-containing copolymer using a mold made of polypropylene, the hydrophobic monomer ends up orienting toward the portion coming into contact with the mold surface. Thus the surface of the lens comprised of copolymer that is obtained exhibits a highly hydrophobic property.
When the lens surface exhibits a hydrophobic property, the lipid component of tears tends to adhere. Thus, noticeable symptoms tend to occur, such as lens fogging and difficulty seeing while wearing contact lenses. Alternatively, proteins and the like further adhere over the lipid component that has attached, potentially causing eye disease. Thus, in silicone-containing copolymers, after polymerizing the lens, imparting a hydrophilic property by a plasma treatment or graft polymerization is known. However, such post-processing is undesirable as a method of mass production because of the number and complexity of the steps required to impart a hydrophilic property to the surface.
In addition to the method of imparting a hydrophilic property by graft polymerization or plasma processing, there is the method of incorporating a high-molecular-weight hydrophilic polymer into the material as an internal moisturizer. This method yields a material containing a high-molecular-weight hydrophilic polymer by polymerizing a polymerization solution obtained by dissolving a high-molecular-weight hydrophilic polymer such as polyvinyl pyrrolidone. The surface of the material that is obtained by this method exhibits a hydrophilic property without a plasma treatment or graft polymerization.
Generally, unpolymerized monomer and oligomer will remain in a polymer obtained by polymerizing a mixed monomer solution. When such a monomer or oligomer elutes out of a lens that is being worn, it may irritate the eye. Thus, alcohol or an alcohol aqueous solution is generally used following polymerization to extract the monomer or oligomer from the polymer. The higher the concentration of the alcohol used in the extraction, the better the extraction efficiency and the more unpolymerized monomer and oligomer that can be removed in a short period. However, the above high-molecular-weight hydrophilic polymer for imparting a hydrophilic property is simply physically entwined in the network structure of the polymer. Thus, when alcohol or a highly concentrated alcohol aqueous solution is used for extraction over an extended period, the high-molecular-weight hydrophilic polymer is extracted from the polymer along with the unreacted monomer or polymer. As a result, it becomes difficult to maintain a good hydrophilic property following extraction.
There is also the method of manufacturing contact lenses with molds of materials other than polypropylene. For example, the use of molds that are molded out of resins having a coefficient of water absorption falling within a range of 0.01 to 0.15 mass percent in the form of cast polymerization resin molds with surfaces having water contact angles falling within a range of 65 to 80 degrees is known. With this method, it becomes possible to manufacture a contact lens with good surface water-wettability because the hydrophilic monomer tends to orient with the portion coming into contact with the mold. However, this method cannot be considered to be a good method. The reason for this is that since the coefficient of water absorption of the constituent resin is high in a resin mold, shape stability following molding is poor or it becomes difficult to look for molding conditions achieving an accurate resin mold.
In light of these circumstances, various methods have been proposed that employ the hydrophilic monomers comprising vinyl groups that are widely employed as starting materials in contact lenses, such as N-vinyl-2-pyrrolidone and N-methyl-N-vinyl acetamide, with the goal of readily improving the hydrophobic property of the surface of soft contact lenses employing silicone-containing copolymers.
WO93/09154 (Japanese Translated PCT Patent Application Publication (TOKUHYO) Heisei No. 7-505169) (Patent Reference 1) discloses a silicone-containing hydrogel obtained by curing a mixed monomer solution obtained by combining at least one vinyl-containing monomer, at least one acrylic-containing monomer, and at least one silicone-containing prepolymer. Examples of the at least one vinyl-containing monomer are N-vinyl-2-pyrrolidone and N-methyl-N-vinylacetamide.
Japanese Unexamined Patent Publication (KOKAI) No. 2011-219512 (Patent Reference 2) discloses a polymerizable composition comprising monomers in the form of: [A] a polymerizable compound containing an acryloyloxy group and not containing silicon atoms, in which the glass transition temperature as a homopolymer is 10° C. or less; [B] a silicone compound containing a polymerizable group; and [C] an N-vinyl pyrrolidinone.
WO2008/061992 (Japanese Translated PCT Patent Application Publication (TOKUHYO) No. 2010-510550) (Patent Reference 3) discloses a silicone-containing copolymer obtained by polymerizing a polymer solution comprising components in the form of: [A] the silicone-containing monomer denoted by general formula I; [B] 3-methacryloyloxypropyltris(trimethylsiloxy)silane; [C] N-vinylpyrrolidone; and [D] at least one additional nonionic hydrophilic monomer.
WO2012/118680 (Patent Reference 4) discloses a silicone-containing copolymer obtained by polymerizing a solution containing: (a) the silicone monomer having a polyethylene glycol group denoted by general formula (I); (b) at least one hydrophilic monomer comprising a vinyl group; and (c) a crosslinking agent having a vinyl group.
US2013/0031873 A1 (Patent Reference 5) discloses a method for manufacturing a silicone-containing copolymer using a mold comprised of a nonpolar resin. The materials used to prepare the lens are described as being a polymerizable silicone monomer and nonsilicone monomer, with the nonsilicone monomer being a monomer having a vinyl group such as N-vinylpyrrolidone or N-vinyl-N-methylacetamide.